In recent years, a plastic film used for packaging food is strongly required to have a vapor and oxygen-proof characteristic (barrier property). Thus, a substrate such as the plastic film is sometimes coated with a transparent SiOx coating in order to provide a high barrier property, and a coating method with high productivity is desired.
As a coating technique of the SiOx coating, conventionally, there are vacuum vapor deposition methods and physical vapor deposition methods (PVD methods) such as a sputtering method. There is also a plasma CVD method serving as an advantageous technique in terms of deposition speed and deposition of a high barrier coating in comparison to these techniques.
As an apparatus using the plasma CVD method for depositing a coating on a surface of a sheet shape substrate, there are apparatuses described in Patent Documents 1 to 3.
The apparatus described in Patent Document 1 is provided with a chamber capable of reducing pressure, and a means for generating plasma in the chamber. The plasma generating means has a contacting and exposing means having an electrode (deposition roller) forming a plasma-facing surface in the chamber, and a confining means provided with a grounded shield (counter electrode) arranged at a position facing the plasma-facing surface of the above electrode and a magnet for generating a magnetic field in which plasma is confined between the grounded shield and the deposition roller. In this plasma processing apparatus, by applying a voltage between the counter electrode and the deposition roller in a state that a source gas is supplied, a glow discharge is generated and the plasma is produced. By causing a film existing on a surface of the deposition roller to pass through in this plasma, a coating layer of SiOx or the like is deposited.
The apparatus described in Patent Document 2 is a magnetron plasma CVD apparatus for depositing a coating while taking up a substrate. In this apparatus, a main roller (deposition roller) serving as a deposition portion and an anode (counter electrode) are arranged so as to face each other in a vacuum chamber, and a magnetic circuit is provided inside the main roller. In this apparatus, plasma is generated on the substrate traveling on the main roller so as to deposit the coating.
The apparatus described in Patent Document 3 is a plasma CVD apparatus for depositing a coating on a surface of a substrate while continuously conveying the substrate in a vacuum chamber. This apparatus has a pair of deposition rollers arranged so as to face each other in parallel or substantially in parallel in such a manner that wound parts of the substrate face each other, magnetic field generating members provided inside the deposition rollers for generating magnetic fields expanded in the vicinity of surfaces of the rollers facing a facing space between the deposition rollers, a plasma power supply in which polarity is reversed between one pole and the other pole, a gas supplying means for supplying a process gas G to the facing space, and a vacuuming means for vacuuming the facing space. The one pole of the plasma power supply is connected to one of the deposition rollers, and the other pole is connected to the other deposition roller.
However, in the plasma processing apparatus disclosed in Patent Document 1, there is a disadvantage that the coating is also formed and deposited on a surface of the counter electrode in contact with the plasma. The insulating coating deposited and grown on the counter electrode covers the surface of the electrode so as to change a characteristic of a glow discharge. Since the coating is deposited on a part other than the film, the source gas is wasted. In addition, the coating deposited by a long-time operation is peeled off from the counter electrode, and this peeled-off coating (coating flake) is attached to a surface of the film so as to cause a coating defect (contamination).
In the CVD apparatus disclosed in Patent Document 2, the counter electrode is installed around the deposition roller around which the film is wound. Thus, a problem that the coating is deposited on the counter electrode is generated as well as the plasma processing apparatus disclosed in Patent Document 1, and an improvement measure is not disclosed.
The plasma CVD apparatus disclosed in Patent Document 3 can solve the “problems caused by contamination of the counter electrode” existing in the apparatuses disclosed in Patent Documents 1 and 2.
That is, in this plasma CVD apparatus, the plasma is generated between the pair of deposition rollers placed side by side in such a manner that axes thereof are parallel to each other, and the coating is deposited on the surface of the film wound around the deposition rollers. Thus, a counter electrode does not exist near the deposition rollers. Therefore, a change in the characteristic of the discharge and generation of the coating flake caused by the deposition on the counter electrode are suppressed, so that long-time stable deposition can be performed.
However, even in the plasma CVD apparatus of Patent Document 3, parts not covered with the film (substrate) exist in both ends in the axial direction of the deposition rollers, and coating deposition on the parts is generated. In this plasma CVD apparatus, the plasma is respectively generated at positions in which the surfaces of the deposition rollers face each other, and the coating is deposited on the film by this plasma. Thus, deposition regions (regions where the plasma is generated) face each other. Therefore, when the coating deposited on one of the deposition rollers is scattered as the coating flake, this coating flake is scattered in the direction of the film wound around the facing other deposition roller and easily attached to this film.